Nanodiamond (ND) also referred to as ultrananocrystalline diamond or ultradispersed diamond (UDD) is a unique nanomaterial which can be produced in hundreds of kilograms by detonation synthesis. These nanodiamonds are called detonation nanodiamonds.
Synthetic nanodiamonds can be produced by several known methods, such as chemical vapour deposition or high pressure.
Detonation nanodiamonds were first synthesized by researchers from the USSR in 1963 by explosive decomposition of high-explosive mixtures with negative oxygen balance in a non-oxidizing medium. A typical explosive mixture is a mixture of trinitrotoluene (TNT) and hexogen (RDX), a preferred weight ratio of TNT/RDX is 40/60.
As a result of the detonation synthesis a diamond-bearing soot also referred to as detonation blend is obtained. This blend comprises nanodiamond particles, which typically have an average particle size of about 2 to 8 nm, and different kinds of non-diamond carbon contaminated by metals and metal oxide particles coming from the material of the detonation chamber and used explosives. The content of nanodiamonds in the detonation blend is typically between 30 and 75% by weight.
The nanodiamond-containing blends obtained from the detonation contain same hard agglomerates, typically having a diameter of above 1 mm. Such agglomerates are difficult to break. Additionally the particle size distribution of the blend is very broad, ranging typically from several to tens of microns.
The diamond carbon comprises sp3 carbon and the non-diamond carbon mainly comprises sp2 carbon species, for example carbon onion, carbon fullerene shell, amorphous carbon, graphitic carbon or any combination thereof.
There are number of processes for the purification of the detonation blends. The purification stage is considered to be the most complicated and expensive stage in the production of nanodiamonds.
For isolating the end diamond-bearing product, use is made of a complex of chemical operations directed at either dissolving or gasifying the impurities present in the material. The impurities, as a rule, are of two kinds: non-carbon (metal oxides, salts etc.) and non-diamond forms of carbon (graphite, black, amorphous carbon).
Chemical purification techniques are based on the different stability of the diamond and non-diamond forms of carbon to oxidants. Liquid-phase oxidants offer an advantage over gas or solid systems, because they allow one to obtain higher reactant concentrations in the reaction zone and, therefore, provide high reaction rates.
In the recent years detonation nanodiamonds have received more and more attention due to several existing applications within electroplating (both electrolytic and electroless), polishing, various polymer mechanical and thermal composites, CVD-seeding, oils and lubricants additives as well as possible new applications such as luminescence imaging, drug delivery, quantum engineering etc. Their usability is based on the fact that the outer surface of detonation nanodiamond, as opposite to for example nanodiamonds derived from micron diamonds by crushing and sieving, is covered with various surface functions.
Recently, use of nanodiamonds in polymers for thermal management has been studied.
Heat generated, for example by electronic devices and circuitry must be dissipated to improve reliability and prevent premature failure. Techniques for heat dissipation can include heat sinks and fans for air cooling, and other forms of cooling such as liquid cooling. Depending on the application, the heat sinks can be made of metal, or ceramic materials, but sometimes also out of polymeric materials. The latter constitute typically thermal greases alike silicones and epoxides thermal interface materials, used typically to adhere the circuits into the device structure itself. When it comes to for example casings of such devices, also thermoplastic thermal composites are used for the overall thermal management throughout the device. Polycarbonate and silicones are also used as LED encapsulants, also an area wherein more efficient thermal management is becoming more and more an issue. Generally, thermoplastic materials are applied in so called secondary polymer based heat sinks and thermosets like silicones in Thermal Interface Materials (TIM), also referred as primary polymer based heat sinks.
The increasing use of polymer materials is based on simple facts of reducing the device weight, and its cost. Moreover, thermally conductive plastics typically boast lower coefficients of thermal expansion (CTE) than for example aluminum and can thereby reduce stresses due to differential expansion, since the plastics more closely match the CTE of silicon or ceramics that they contact. If the contact between the thermal compound and silicon/ceramics surface is altered, that will have an adverse effect on the component function and life-time. Polymer composites offer also design freedom for molded-in functionality and parts consolidation; and they can eliminate costly post-machining operations. The use of polymeric materials is however limited by their native thermal conductivity properties, reaching typically thermal conductivity values of only around 0.2 W/mK.
For example, miniaturization of electronic chips has become an important topic for development of integrated circuit. Because sizes of electronic elements become smaller, and their operating speeds become faster, how to dissipate the heat generated by an electronic element during operation so as to maintain its working performance and stability has become one of the points for research.
Several methods to improve polymer thermal conductivity properties have been presented. If preparing thermal solutions that are at the same time electrically insulating, the present solutions are based on use of various ceramic particles, including alumina, hexagonal and cubic boron nitride, aluminum nitride, carbides like boron carbide etc. In solutions being at the same thermally and electrically insulating, various forms of graphitic and amorphous carbon including graphite, graphene, carbon fibers, pyrolytic carbon, carbon nanotubes, micron diamonds and nanodiamonds derived from micron sized diamonds via crushing and sieving are applied as thermal filler. Also metal particles alike various sized silver particles are readily applied for making high efficiency thermal management solutions.
If the added filler materials cannot be distributed evenly into the polymer matrix, but are forming heavy agglomerates in the produced matrix, the use of additives may also result in poorer mechanical and thermal properties as in initial, native polymer material. This problem gets more and more severe, the higher the total content of various fillers in a ready polymer composite is rising. Moreover, the higher the content of various inorganic or metallic particles, the higher is the compound cost, the higher is the wear of used processing tools, and the higher is the weight of produced compounds.
As the polymer based heat sink thermal conductivity is dependent on the used materials thermal conductivity but also on the boundary thermal conductivity between the parent polymer and added thermal filler particles, the wetting or coupling effect between the thermal particle surface and parent polymer material is having a big impact on received compound thermal conductivities. It is commonly known that the especially the present ceramic and metal fillers exhibit remarkably inert surface properties having an impaired effect on said wetting or coupling of particles into parent polymer(s), having an adverse effect on received compound thermal conductivities.
Typically, thermal conductivity is measured both in-plane and through-plane of a material, the in-plane conductivity featuring normally higher thermal conductivity values than the through-plane conductivity. Generally, anisotropic fillers such as hexagonal boron nitride and graphite are applied for improving the in-plane thermal conductivities and isotropic, spherical fillers such as alumina particles to improve thermal conductivities both in in-plane and through-plane directions. The used fillers are selected according to application need.
In-plane and through-plane thermal conductivities can be determined by Laser Flash method. The other method measuring the thermal conductivity is by Hot Disk method, giving a value for the average thermal conductivity only.
The electric properties of thermal composites can be tuned by selecting either dielectric or electrically conducting filler additives. Typically, for generating the phonon percolation throughout the thermal composite, the additive total concentrations are very high, starting from 20% but exceeding also concentrations of 80%. Some of the most advanced thermal composites can contain several of the above mentioned fillers.
There are upper limits on present thermal conductivities, and it is difficult to improve these further due to already extremely high filler contents. Excess filler content is detrimental for the polymer composite's other important properties, such as mechanical properties and weight. When certain filler loading is exceeded, the used polymer loses its wetting ability and the compound breaks up into powder or fragmented pieces.
Therefore, there have been attempts to replace a portion of the filler materials with nanodiamonds for improving thermal conductivities of polymer composites.
US 2010/0022423 A1 discloses use of nanodiamonds to increase thermal conductivity in a polymeric material (polymeric grease). The nanodiamond thermal grease comprises nanodiamond powder, thermal powder and a substrate. The nanodiamond powder has volume percentage of 5% to 30%, the thermal powder of 40% to 90%, and the substrate of 5% to 30%.
The polymeric grease disclosed in US 2010/0022423 A1 has high nanodiamond and thermal powder (filler) content, and low substrate content. Because the filler and nanodiamond content compared to the substrate content is high the disadvantages of high filler content are still present.
US 2010/0022423 A1 further discloses a method for manufacturing nanodiamond thermal grease. First a substrate is heated, and then nanodiamond powder is put into the preheated substrate. A disperser is used to disperse the nanodiamond powder in the substrate. Thermal powder is put into the mixture of the substrate and nanodiamond, and the mixture is blended to form nanodiamond thermal grease.
US 2013/0206273 A1 describes a polymer matrix nanocomposite and a method for manufacturing the nanocomposite. Nanoparticle such as nanodiamond may be formulated as a solution or dispersion and cast or coated, or mechanically dispersed into a polymer resin matrix. The nanoparticle may also be blended with filler particles such as mica and carbon black. Blending and dispersion of the filler and the polymer resin may be accomplished by methods such as extrusion, shear mixing, three roll milling etc.
With the methods of US 2010/0022423 A1 and US 2013/0206273 A1 it is difficult to evenly distribute the nanodiamond particles and the filler into the polymer matrix. Uneven distribution leads to poor properties such as thermal conductivities and mechanical properties.
Publication H. Ebadi-Dehaghani, M. Nazempour, Thermal Conductivity of Nanoparticles Filled Polymers, Smart Nanoparticles Technologies, 2012, 519-540 discusses thermal conductivity of nanoparticles filled polymers. Improved thermal conductivities of polymers filled with nanosized filler materials, such as graphite, boron nitride and carbon nanotubes, are disclosed in the publication. However, nanodiamonds are not mentioned in the publication as a filler material for improving thermal conductivity of polymers.
Based on above there is a need for a nanodiamond containing thermal composite having improved thermal conductivities, preferably without impaired other important properties. There is an identified need to lower the overall weight of the thermal composites and to reduce the wear of processing tools used for composite manufacturing. Moreover, especially in solutions applied in automotive and other means of transportation, there is a simultaneous need to improve the manufactured composite mechanical properties. Further, the importance of polymer thermal compounds within automotive and other means of transportation is getting more and more pronounced as these devices are electrically powered. Within automotive sector, there is an emerging need to find solutions related especially to E-drive and used battery solutions.
In a preferred case, the nanodiamond containing thermal composites should bring the same or improved thermal properties with reduced overall composite production cost. Hence, the nanodiamond containing thermal composites should be available with remarkably low nanodiamond additions.
Moreover, there is a need for an improved process for producing nanodiamond containing thermal composites having improved thermal conductivities. The nanodiamond containing thermal composites should also be easy and cheap to manufacture and readily adaptable to various thermal applications.